Bently Nevada 991-25-70-01-05 Thrust Transmitter

The Bently Nevada 991-25-70-01-05 is a 2-wire loop-powered thrust (axial displacement) transmitter developed for OEM machinery monitoring applications on compressors, pumps, motors, and rotating equipment. It converts the proximity probe signal into a standard 4–20 mA output for direct integration with PLC, DCS, or control systems.

Ordering Information

Key Features

• Integrated proximity probe driver circuit
• 4–20 mA proportional axial displacement output
• Non-interacting Zero and Span adjustments
• Power-up signal suppression to avoid false trips
• “Not OK” signal blanking diagnostics
• Dedicated PROX OUT dynamic signal connector
• Encapsulated construction for high-humidity environments

Specifications

Installation Steps

Physical Mechanical Mounting

• Snap the transmitter onto a standard 35 mm DIN rail
• Install away from high-temperature sources
• Use a 304 stainless steel enclosure for outdoor environments

Probe Installation & Gapping

• Use only matched 3300 NSv probe systems
• Maintain a total system cable length of exactly 7.0 meters
• Verify the shaft target area meets minimum sizing requirements
• Adjust the probe to the midpoint of the linear operating range
• Confirm the PROX OUT voltage is approximately 6.5 Vdc

Electrical Loop Wiring

• Connect the transmitter to a +12 to +35 Vdc loop supply
• Verify Power (+) and Signal (-) polarity before energizing
• Tighten the miniature coaxial connector securely
• Ground the cable shield at the control cabinet side only

Zero & Span Calibration

• Adjust the Zero potentiometer at the mechanical center position
• Set loop output current to 12 mA during midpoint calibration
• Adjust the Span potentiometer at full-scale displacement
• Non-interacting potentiometers minimize recalibration iterations

FAQ

Why is the initial probe gap important?

• Incorrect gaps limit usable measurement range
• Improper installation may allow probe-to-rotor contact
• The operating gap must remain within 0.25 to 1.65 mm (10 to 65 mils)

Why does the loop current drop below 3.6 mA?

• The transmitter has entered “Not OK” protection mode
• Probe or extension cable may be open or shorted
• The coaxial connector may not be fully seated
• The failsafe output prevents false PLC or DCS trip signals

Can an oscilloscope connect directly to the PROX OUT port?

• The PROX OUT connector is non-isolated from the 4–20 mA loop
• Standard grounded oscilloscopes can create ground loops
• Use a battery-powered meter or isolated interface adapter instead

Can the extension cable be shortened or spliced?

• Modifying cable length is strictly prohibited
• Cable impedance changes affect calibration accuracy
• Shortening or extending the cable invalidates factory calibration

Compatibility Notes

The transmitter is specifically designed for use with:

• Bently Nevada 3300 NSv proximity probe systems
• OEM centrifugal compressors
• Small pumps and rotating machinery
• PLC and DCS analog input systems